bmetfandomcom-20200215-history
Telecommunications
Telecommunication is the assisted transmission over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as the telephone, television, radio or computer. Early inventors in the field of telecommunication include Alexander Graham Bell, Guglielmo Marconi and John Logie Baird. Telecommunication is an important part of the world economy and the telecommunication industry's revenue was estimated to be $1.2 trillion in 2006. Key concepts |The word telecommunication was adapted from the French word télécommunication. It is a compound of the Greek prefix tele-'' (τηλε-), meaning 'far off', and the Latin ''communicare, meaning 'to share'.Telecommunication, tele-'' and ''communication, New Oxford American Dictionary (2nd edition), 2005. The French word télécommunication was coined in 1904 by French engineer and novelist Édouard Estaunié.Jean-Marie Dilhac, From tele-communicare to Telecommunications, 2004. |} A number of key concepts reoccur throughout the literature on modern telecommunication systems. Some of these concepts are discussed below. Basic elements A basic telecommunication system consists of three elements: * a transmitter that takes information and converts it to a signal; * a transmission medium that carries the signal; and, * a receiver that receives the signal and converts it back into usable information. For example, in a radio broadcast the broadcast tower is the transmitter, free space is the transmission medium and the radio is the receiver. Often telecommunication systems are two-way with a single device acting as both a transmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a telephone line is called point-to-point communication because it is between one transmitter and one receiver. Point-to-point telecommunications generally refers to a connection restricted to two endpoints. Telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers. Analog or digital Signals can be either analog or digital. In an analog signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (for example ones and zeros). During transmission the information contained in analogue signals will be degraded by noise. Conversely, unless the noise exceeds a certain threshold, the information contained in digital signals will remain intact. Noise resistance represents a key advantage of digital signals over analog signals. Networks A network is a collection of transmitters, receivers and transceivers that communicate with each other. Digital networks consist of one or more routers that work together to transmit information to the correct user. An analog network consists of one or more switches that establish a connection between two or more users. For both types of network, repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise.ATIS Telecom Glossary 2000, ATIS Committee T1A1 Performance and Signal Processing (approved by the American National Standards Institute), 28 February 2001. Channels A channel is a division in a transmission medium so that it can be used to send multiple streams of information. For example, a radio station may broadcast at 96.1 MHz while another radio station may broadcast at 94.5 MHz. In this case, the medium has been divided by frequency and each channel has received a separate frequency to broadcast on. Alternatively, one could allocate each channel a recurring segment of time over which to broadcast—this is known as time-division multiplexing and is sometimes used in digital communication. Modulation The shaping of a signal to convey information is known as modulation. Modulation can be used to represent a digital message as an analog waveform. This is known as keying and several keying techniques exist (these include phase-shift keying, frequency-shift keying and amplitude-shift keying). Bluetooth, for example, uses phase-shift keying to exchange information between devices.Haykin, pp 344-403.Bluetooth Specification Version 2.0 + EDR (p 27), Bluetooth, 2004. Modulation can also be used to transmit the information of analogue signals at higher frequencies. This is helpful because low-frequency analogue signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analogue signal must be superimposed on a higher-frequency signal (known as the carrier wave) before transmission. There are several different modulation schemes available to achieve this (two of the most basic being amplitude modulation and frequency modulation). An example of this process is a disc jockeys voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel “96 FM”).Haykin, pp 88-126. Further reading * Wikibooks: Signals and Systems References Video thumb|300px|right|Telecommunications Category:Telecommunication Category:Electronics Category:Biomedical Engineering Technology